This paper introduced a dual description logic for relations, in order to represent the difference of tuples and attribute values. In the logic, concepts are classified into two classes as follows: tuple-concepts and attribute-value concepts. This paper firstly gives the logical language, syntax and semantics of the logic, and secondly defines a transformation of a relation in a database into a knowledge base in the dual description logic, and last proposes a transformation of a model for the dual description logic into a relation.